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SubjectRe: [PATCH linux-next v2] SUNRPC: rpcrdma_register_default_external: Dynamically allocate ib_phys_buf
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On Mon, 2013-03-11 at 15:15 -0400, J. Bruce Fields wrote:
> On Mon, Mar 11, 2013 at 12:51:44PM -0600, Tim Gardner wrote:
> > On 03/11/2013 12:14 PM, J. Bruce Fields wrote:
> > <snip>
> > >>
> > >> v2 - Move the array of 'struct ib_phys_buf' objects into struct rpcrdma_req
> > >> and pass this request down through rpcrdma_register_external() and
> > >> rpcrdma_register_default_external(). This is less overhead then using
> > >> kmalloc() and requires no extra error checking as the allocation burden is
> > >> shifted to the transport client.
> > >
> > > Oh good--so that works, and the req is the right place to put this? How
> > > are you testing this?
> > >
> > > (Just want to make it clear: I'm *not* an expert on the rdma code, so my
> > > suggestion to put this in the rpcrdma_req was a suggestion for something
> > > to look into, not a claim that it's correct.)
> > >
> >
> > Just compile tested so far. Incidentally, I've been through the call stack:
> >
> > call_transmit
> > xprt_transmit
> > xprt->ops->send_request(task)
> > xprt_rdma_send_request
> > rpcrdma_marshal_req
> > rpcrdma_create_chunks
> > rpcrdma_register_external
> > rpcrdma_register_default_external
> >
> > It appears that the context for kmalloc() should be fine unless there is
> > a spinlock held around call_transmit() (which seems unlikely).
>
> Right, though I think it shouldn't be GFP_KERNEL--looks like writes
> could wait on it.

Nothing inside the RPC client should be using anything heavier than
GFP_NOWAIT (unless done at setup).

> In any case, the embedding-in-rpcrdma_req solution does look cleaner if
> that's correct (e.g. if we can be sure there won't be two simultaneous
> users of that array).

Putting it in the rpcrdma_req means that you have one copy per transport
slot. Why not rather put it in the rpcrdma_xprt?
AFAICS you only need this array at transmit time for registering memory
for RDMA, at which time the transport XPRT_LOCK guarantees that nobody
else is competing for these resources.

--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com


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